A retroelement-encoded effector of Blumeria graminis targets barley RACB for exploitation of a host cell developmental pathway
R. HÜCKELHOVEN (1), T. Reiner (2), B. Scheler (2), V. Schnepf (2), B. Zechmann (3), M. Nottensteiner (2) (1) Technische Universität München, Germany; (2) TU München, Germany; (3) Center for Microscopy and Imaging at Baylor University in Waco, Texas, U.S.A.

Blumeria graminis, as a haustorium-forming ectoparasite, has to overcome pre- and post-invasive host immunity and additionally has to reprogram the host for changes in host cell architecture and physiology to establish biotrophy. Little is known about virulence functions that target host cell development or physiology. The signalling G-protein RACB is a barley susceptibility factor and a regulator of host cytoskeleton dynamics. RACB supports fungal invasion and haustorium expansion. It is negatively controlled by MICROTUBULE-ASSOCIATED ROP GTPASE ACTIVATING PROTEIN1 and cytoplasmic ROP-BINDING KINASE1. However, the physiological function of RACB is not fully understood. We show that RACB does not function as a negative regulator of MAMP-triggered immune responses in barley. Instead, RACB is required for polar cell development. Silencing of RACB results in defects in polar root hair outgrowth and in asymmetric cell development of stomata complexes. Strikingly, we found that B. graminis expresses an effector called ROP-interactive peptide 1 (ROPIP1) from a non-autonomous retroelement. ROPIP1 interacts with RACB in planta, is translocated into the host cytoplasm and destabilizes host microtubules. ROPIP1 has virulence supporting functions when over-expressed in the host, and host-induced gene silencing of ROPIP1 limits fungal success. At the genomic level, ROPIP1 has thousands of paralogs, of which several might have the potential for neo-functionalization as effector genes.

Abstract Number: P7-174
Session Type: Poster